﻿<h1>Memory Leaking Scenarios</h1>

<p>
When programming in a language supporting auto garbage collection,
generally we don't need care about memory leaking problems,
for the runtime will collect unused memory regularly.
However, we do need to be aware of some special scenarios
which may cause kind-of or real memory leaking.
The remaining of the current article will list several such scenarios.
</p>

<a class="anchor" id="substring"></a>
<h3>Kind-of Memory Leaking Caused by Substrings</h3>

<div>
<p>
Go specification doesn't specify whether or not the result string
and base string involved in a substring expression should share
the same underlying <a href="memory-block.html">memory block</a> to host
<a href="string.html">the underlying byte sequences</a> of the two strings.
The standard Go compiler/runtime does let them share the same underlying memory block.
This is a good design, which is both memory and CPU consuming wise.
But it may cause kind-of memory leaking sometimes.
</p>

For example, after the <code>demo</code> function in the following example is called,
there will be about 1M bytes memory leaking (kind of),
until the package-level variable <code>s0</code> is modified again elsewhere.

<pre class="line-numbers"><code class="language-go">var s0 string // a package-level variable

// A demo purpose function.
func f(s1 string) {
	s0 = s1[:50]
	// Now, s0 shares the same underlying memory block
	// with s1. Although s1 is not alive now, but s0
	// is still alive, so the memory block they share
	// couldn't be collected, though there are only 50
	// bytes used in the block and all other bytes in
	// the block become unavailable.
}

func demo() {
	s := createStringWithLengthOnHeap(1 << 20) // 1M bytes
	f(s)
}
</code></pre>

<p>
</p>

To avoid this kind-of memory leaking, we can convert the substring to a
<code>[]byte</code> value then convert the <code>[]byte</code> value back
to <code>string</code>.

<pre class="line-numbers"><code class="language-go">func f(s1 string) {
	s0 = string([]byte(s1[:50]))
}
</code></pre>

<p>
The drawback of the above way to avoid the kind-of memory leaking
is there are two 50-byte duplicates which happen in the conversion process,
one of them is unnecessary.
</p>

We can make use of one of <a href="string.html#conversion-optimizations">the
optimizations</a> made by the standard Go compiler to avoid the unnecessary duplicate,
with a small extra cost of one byte memory wasting.

<pre class="line-numbers"><code class="language-go">func f(s1 string) {
	s0 = (" " + s1[:50])[1:]
}
</code></pre>

<p>
The disadvantage of the above way is the compiler optimization may become
invalid later, and the optimization may be not available from other compilers.
</p>

The third way to avoid the kind-of memory leaking is to utilize
the <code>strings.Builder</code> supported since Go 1.10.

<pre class="line-numbers"><code class="language-go">import "strings"

func f(s1 string) {
	var b strings.Builder
	b.Grow(50)
	b.WriteString(s1[:50])
	s0 = b.String()
}
</code></pre>

<p>
The disadvantage of the third way is it is a little verbose (by comparing to the first two ways).
A good news is, since Go 1.12,
we can call the <code>Repeat</code> function with the <code>count</code> argument as <code>1</code>
in the <code>strings</code> standard package to clone a string.
Since Go 1.12, the underlying implementation of <code>strings.Repeat</code> will make use of <code>strings.Builder</code>,
to avoid one unnecessary duplicate.
</p>

</div>

<a class="anchor" id="subslice"></a>
<h3>Kind-of Memory Leaking Caused by Subslices</h3>

<div>
Similarly to substrings, subslices may also cause kind-of memory leaking.
In the following code, after the <code>g</code> function is called,
most memory occupied by the memory block hosting the elements of <code>s1</code>
will be lost (if no more values reference the memory block).

<pre class="line-numbers"><code class="language-go">var s0 []int

func g(s1 []int) {
	// Assume the length of s1 is much larger than 30.
	s0 = s1[len(s1)-30:]
}
</code></pre>

<p>
</p>

If we want to avoid the kind-of memory leaking, we must duplicate the
30 elements for <code>s0</code>, so that the aliveness of <code>s0</code>
will not prevent the memory block hosting the elements of <code>s1</code>
from being collected.

<pre class="line-numbers"><code class="language-go">func g(s1 []int) {
	s0 = append([]int(nil), s1[len(s1)-30:]...)
	// Now, the memory block hosting the elements
	// of s1 can be collected if no other values
	// are referencing the memory block.
}
</code></pre>

</div>

<a class="anchor" id="dead-slice-elements"></a>
<h3>Kind-of Memory Leaking Caused by Not Resetting Pointers in Lost Slice Elements</h3>

<div>
In the following code, after the <code>h</code> function is called,
the memory block allocated for the first and the last elements of slice
<code>s</code> will get lost.

<pre class="line-numbers"><code class="language-go">func h() []*int {
	s := []*int{new(int), new(int), new(int), new(int)}
	// do something with s ...

	return s[1:3:3]
}
</code></pre>

<p>
As long as the returned slice is still alive, it will prevent any elements
of <code>s</code> from being collected, which in consequence prevents
the two memory blocks allocated for the two <code>int</code> values
referenced by the first and the last elements of <code>s</code> from being collected.
</p>

If we want to avoid such kind-of memory leaking,
we must reset the pointers stored in the lost elements.

<pre class="line-numbers"><code class="language-go">func h() []*int {
	s := []*int{new(int), new(int), new(int), new(int)}
	// do something with s ...

	// Reset pointer values.
	s[0], s[len(s)-1] = nil, nil
	return s[1:3:3]
}
</code></pre>

<p>
</p>

<p>
We often need to reset the pointers for some old slice elements in
<a href="container.html#slice-manipulations">slice element deletion operations</a>.
</p>

</div>

<a class="anchor" id="hanging-goroutine"></a>
<h3>Real Memory Leaking Caused by Hanging Goroutines</h3>

<div>
<p>
Sometimes, some goroutines in a Go program may stay in blocking state for ever.
Such goroutines are called hanging goroutines.
Go runtime will not kill hanging goroutines, so the resources allocated for
(and the memory blocks referenced by) the hanging goroutines will never get garbage collected.
</p>

<p>
There are two reasons why Go runtime will not kill hanging goroutines.
One is that sometimes it is hard for Go runtime to judge whether or not a blocking goroutine will be blocked for ever.
The other is sometimes we deliberately make a goroutine hanging.
For example, sometimes we may let the main goroutine of a Go program hang to avoid the program exiting.
</p>

<p>
We should avoid hanging goroutines which are caused by some logic mistakes in code design.
</p>

</div>

<h3>Real Memory Leaking Caused by Not Stopping <code>time.Ticker</code> Values Which Are Not Used Any More</h3>

<p>
When a <code>time.Timer</code> value is not used any more,
it will be garbage collected after some time.
But this is not true for a <code>time.Ticker</code> value.
We should stop a <code>time.Ticker</code> value when it is not used any more.
</p>

<h3>Real Memory Leaking Caused by Using Finalizers Improperly</h3>

<div>
<p>
Setting a finalizer for a value which is a member of a cyclic reference group may
<a href="https://golang.org/pkg/runtime/#SetFinalizer">prevent all memory blocks allocated
for the cyclic reference group from being collected</a>.
This is real memory leaking, not kind of.
</p>

For example, after the following function is called and exits,
the memory blocks allocated for <code>x</code> and <code>y</code> are not
guaranteed to be garbage collected in future garbage collecting.

<pre class="line-numbers"><code class="language-go">func memoryLeaking() {
	type T struct {
		v [1<<20]int
		t *T
	}

	var finalizer = func(t *T) {
		 fmt.Println("finalizer called")
	}

	var x, y T

	// The SetFinalizer call makes x escape to heap.
	runtime.SetFinalizer(&x, finalizer)

	// The following line forms a cyclic reference
	// group with two members, x and y.
	// This causes x and y are not collectable.
	x.t, y.t = &y, &x // y also escapes to heap.
}
</code></pre>

<p>
So, please avoid setting finalizers for values in a cyclic reference group.
</p>

<p>
By the way, we <a href="unofficial-faq.html#finalizers">shouldn't use
finalizers as object destructors</a>.
</p>
</div>

<h3>Kind-of Resource Leaking by Deferring Function Calls</h3>

<div>

<p>
Please read <a href="defer-more.html#kind-of-resource-leaking">this article</a> for details.
</p>

</div>


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